diff --git a/common.mk b/common.mk
index 6183af9be2..17116c4c47 100644
--- a/common.mk
+++ b/common.mk
@@ -180,7 +180,7 @@ EXTMK_ARGS    =	$(SCRIPT_ARGS) --extension $(EXTS) --extstatic $(EXTSTATIC) \
 		--make-flags="V=$(V) MINIRUBY='$(MINIRUBY)'" \
 		--gnumake=$(gnumake) --extflags="$(EXTLDFLAGS)" \
 		--
-INSTRUBY      =	$(SUDO) $(RUNRUBY) -r./$(arch)-fake $(tooldir)/rbinstall.rb
+INSTRUBY      =	$(SUDO) $(INSTRUBY_ENV) $(RUNRUBY) -r./$(arch)-fake $(tooldir)/rbinstall.rb
 INSTRUBY_ARGS =	$(SCRIPT_ARGS) \
 		--data-mode=$(INSTALL_DATA_MODE) \
 		--prog-mode=$(INSTALL_PROG_MODE) \
diff --git a/defs/gmake.mk b/defs/gmake.mk
index f093fd4e26..316ff18c20 100644
--- a/defs/gmake.mk
+++ b/defs/gmake.mk
@@ -6,6 +6,10 @@ override mflags := $(filter-out -j%,$(MFLAGS))
 MSPECOPT += $(if $(filter -j%,$(MFLAGS)),-j)
 nproc = $(subst -j,,$(filter -j%,$(MFLAGS)))
 
+ifneq ($(filter %darwin%,$(arch)),)
+INSTRUBY_ENV += SDKROOT=/
+endif
+
 CHECK_TARGETS := great exam love check test check% test% btest%
 # expand test targets, and those dependents
 TEST_TARGETS := $(filter $(CHECK_TARGETS),$(MAKECMDGOALS))